1. Introduction
Isosulphan Blue is a reagent used in diagnostic kits for detecting cancer cells.
Isosulphan Blue (Molecular formula: C27H31N2NaO6S2) is also known as sodium salt of N-[4-[4-(diethylamino)phenyl](2,5-disulfophenyl)methylene]2,5-cyclohexadien-1-ylidene]-N-ethyl-ethanaminium hydroxide.
Chemical Structure:

2. Prior Art
Isosulphan Blue has been manufactured by using 4-formyl benzene-1,3-disulphonic acid and Diethylaniline followed by oxidation with lead oxide or potassium dichromate as an oxidizing agent.
U.S. Pat. No. 1,531,507 has disclosed a scheme in which the intermediate 4-formyl benzene-1,3-disulphonic acid sodium salt is prepared by sulphonation of orthochlorobenzaldehyde using oleum 26% and oleum 65% followed by reaction with sodium sulphite.
The process disclosed in the above mentioned patent is shown in scheme I below:

However, oxidising agents like lead oxide or potassium dichromate lead to over oxidized products.
Furthermore, traces of lead remaining in the final compound can produce toxic effects (lead poisoning) in the recipients.
This is a significant limitation of the prior art manufacturing process mentioned above.